Many magnetic resonance (MR) sequences generate data which contains signals in multiple coherence pathways representing groups of protons having substantially the same proton spin precession angle. Usually only the signal in one certain coherence pathway is desired. If this signal cannot be isolated, or the other unwanted signals cannot be suppressed, errors occur in the results. Many magnetic resonance (MR) pulse sequences contain multiple radio-frequency (RF) pulses to prepare the magnetization before acquiring the data. Some examples include spectroscopy techniques such as stimulated echo acquisition mode (STEAM) single voxel spectroscopy, point resolved spectroscopy (PRESS) and double-quantum filtered (DQF) spectroscopy, and imaging techniques such as displacement-encoding with stimulated echoes (DENSE) and harmonic phase analysis (HARP). In the acquired data of these sequences, signals from multiple coherence transfer pathways (CTPs) exist. Depending on the applications, usually only the signal in one certain coherence pathway is desired. If this signal cannot be isolated, or the other unwanted signals cannot be eliminated, then the acquired data is contaminated as ghosting artifacts and poor signal-to-noise ratio (SNR) can be seen and this leads to inaccurate results.
Known systems use phase cycling to address this problem, where desired signals add coherently and unwanted signals add destructively. This selective constructive or destructive addition can be achieved by carefully cycling the phases of the RF pulses to particular values. In the initial implementations of phase cycling, data from various imaging scans is averaged by saving data in the same memory location. Therefore, at the end of acquisition, various original phase cycling scans cannot be inspected separately. Moreover, only the desired signal in certain coherence pathway is available. The signals in other coherence pathways cannot be inspected, although in most cases there is no need to inspect them.
Known systems also use phase rotation to address shortcomings of phase cycling in localized spectroscopy. In phase rotation, the phases of the RF pulses are still carefully set to cycle among particular values, and data from various imaging scans is stored as rows in a two-dimensional (2D) matrix. After the data acquisition, a one-dimensional (1D) Fourier transform is performed in the column direction on this matrix, and a specific row of this transformed matrix is extracted to represent the 1D free induction decay (FID) containing only the desired signal in a certain coherence pathway, while some other particular rows contain the signals in the unwanted coherence pathways. Other post-processing such as zero-padding and frequency/phase correction may be applied on the rows, and followed by a 1D Fourier transform in the row direction to obtain a resultant 2D matrix, having rows comprising 1D spectra of the corresponding signals in different coherence pathways. Phase rotation therefore enables inspection of spectra of the separated signal in different coherence pathways at the same time by observing an overall view represented by a resultant 2D matrix.
Some imaging techniques, such as displacement-encoding with stimulated echoes (DENSE) and harmonic phase analysis (HARP), have a similar need to isolate the specific signal in one certain pathway. Known systems use variants of phase cycling methods to isolate desired stimulated echo signals. In known systems, phase cycling steps need to achieve the effect that desired signals add coherently and unwanted signals add destructively and planning phase cycling methods is difficult. In addition, known phase cycling methods typically use a simple average to only acquire a desired signal, and are unable to concurrently acquire unwanted signals. A system according to invention principles addresses these deficiencies and related problems.